Method of joining self-aligning optical fibers

ABSTRACT

A method of forming an optical fiber joint between a pair of elongated optical fibers each of which has a longitudinal axis surrounded by a core and cladding having different refractive indices and forming a single-mode light-guiding region, the core and cladding of each fiber having non-circular transverse cross-sections defining two polarization-maintaining axes of birefringence transverse to the longitudinal axis of the fiber. Each of the fibers also has predetermined external flat reference surfaces for locating the core and cladding and the axes of birefringence within each fiber from the exterior geometry of the fiber. The method comprises shaping an elongated glass preform to have a cross-sectional configuration with flat longitudinal surfaces extending along the preform length, which flat surfaces are large-scale complements of the external reference surfaces on the fibers to be joined, such that the preform longitudinal surfaces will mate with the fiber reference surfaces when the preform is drawn down to the scale of the fibers; heating the shaped preform to a softened condition and then drawing the softened preform in a longitudinal direction to reduce the scale of the complementary surfaces so that the cross-sectional configuration of the drawn preform matches the cross-sectional configuration of the external reference surfaces on the fibers to be joined, and so that the preform longitudinal surfaces and the fiber reference surfaces will mate together; cooling the drawn preform and placing the optical fibers to be joined thereon with the ends of the fibers butted together, with the external reference surfaces on the fibers matingly engaging the complementary longitudinal surfaces on the drawn preform; and bonding the fibers to the drawn preform and bonding the abutting ends of the fibers to each other.

This application is a continuation of my patent application Ser. No.077,388 filed July 24, 1987 and now abandoned, which in turn is adivisional of my application Ser. No. 778,407 filed Sept. 20, 1985, U.S.Pat. No. 4,755,021, for "Self-Aligning Optical Fiber and Fiber-RingOptical Resonator Using Same," which in turn is a Continuation-In-Partof my application Ser. No. 469,977 filed Feb. 25, 1983, U.S. Pat. No.4,697,876, for "Fiber-Optic Rotation Sensor," which in turn is aContinuation-In-Part of my application Ser. No. 404,283 filed Aug. 2,1982, U.S. Pat. No. 4,669,814, for "Self-Aligning Optical Fiber withAccessible Guiding Region."

FIELD OF THE INVENTION

The present invention relates generally to the field of fiber optics,and particularly single-moded and dual-polarization-preservingfiber-optic waveguides.

DESCRIPTION OF RELATED ART

Fiber optics is generally concerned with the transmission of light alonga transparent fiber structure which has a higher refractive index thanits surroundings. Currently it is possible to manufacture long,continuous strands of optical fiber which may propagate signals withoutsubstantial attenuation over long distances. It is also possible tomanufacture the fiber structure as an optical waveguide wherein onlypreselected modes of light propagate in the fiber. By limiting wavepropagation through the fiber to a single mode, the bandwidth of theoptical fiber may be exceedingly high to provide a highinformation-transfer capacity.

While the development of optical fibers for telecommunications systemsis becoming rather highly advanced, the use of fiber optics for sensingand control systems is still in its early development. In sensing andcontrol systems, a fiber-optic transducer is used that exploits eithermulti-mode or single-mode light propagation in an optical fiber.

While multi-mode sensors use amplitude variations in the optical signalsto sense and transmit the desired information, single-mode sensors usephase variations rather than amplitude variations. The single-modesensors usually involve mechanisms for altering such properties of thefiber as path length or index of refraction to effect the desired phasevariations in the optical signal. In the case of the fiber-opticgyroscopy, the single-mode sensor measures acceleration which inherentlyalters the propagation of light even though the fiber is not affected.Thus, in contrast to multi-mode sensors, in single-mode sensors theuniformity and mechanism of light propagation and hence thecharacteristics of the fiber are especially critical.

Single-mode sensors and fiber components such as directional couplers,are sensitive to the state of polarization of the light in the fiber.Thus, for single-mode transducers, it is desirable to useelliptical-core or other kinds of polarization-holding fiber. See, e.g.,McMahon et al, "Fiber-Optic Transducers," IEEE Spectrum, December 1981,pages 24-27. Most of these polarization-holding fibers are capable ofpreserving the polarization of signals along two different, usuallyorthogonal, axes, such as the major and minor axes of an ellipticalcore.

There are well-known techniques for making long, continuous, single-modeoptical fibers. Keck et al. U.S. Pat. No. 3,711,262 issued Jan. 16,1973, for example, describes the conventional method of producing anoptical waveguide by first forming a film of glass with a preselectedindex of refraction on the inside wall of a glass tube, and then drawingthe glass-film combination to reduce the cross-sectional area and tocollapse the film of glass to form a fiber having a solid cross-section.As a result of this process, the core is formed from the glass film, andthe cladding is formed from the glass tube.

It is also known that multiple core and cladding layers may be depositedon the inside of a preform which is then collapsed and drawn, so thatthe preform tube becomes a support jacket around the core and claddinglayers. Light propagated through a fiber formed in this manner isconfined to the guiding region formed by the core and cladding layersand does not significantly interact with the support jacket.Consequently the optical properties of the support jacket can beconsiderably inferior to the optical qualities of the core and cladding.Details of this process for forming multiple core and cladding layersare disclosed in MacChesney et al., "A New Technique for the Preparationof Low-Loss and Graded-Index Optical Fibers," Proceedings of the IEEE,62, at 1280 (1974), and Tasker and Ench, "Low-Loss Optical Waveguideswith Pure Fused SiO₂ Cores," Proceedings of the IEEE, 62, at 1281(1974).

It is known that elliptical-core, polarization-preserving optical fibersmay be drawn from elliptical-core preforms. The preforms may bemanufactured by collapsing a cylindrical preform or tube, with a slightvacuum in the center. Another method of manufacturing an elliptical-corepreform is to fabricate the substrate tube to have a wall of non-uniformthickness and then collapse the tube by heating it to the softeningpoint. The surface tension in the shaped wall, which occurs during thecollapsing and subsequent drawing steps, causes the resulting fiber coreto be elliptical in cross-section, See, e.g., Pleibel et al. U.S. Pat.No. 4,274,854 issued June 23, 1981.

As is known in the literature, e.g., Dyott et al., "Preservation ofPolarization in Optical-Fiber Waveguides with Elliptical Cores",Electronics Letters, June 21, 1979, Vol. 15, No. 13, pp. 380-382, fiberswith elliptical cores and a large index difference between the core andcladding preserve the polarization of fundamental modes aligned with thelong and short axes of the ellipse, i.e., modes having their electricfields parallel to the major and minor axes of the ellipse. If thecore-cladding index difference and the difference between the lengths ofthe major and minor axes of the ellipse are sufficiently large to avoidcoupling of the two fundamental modes, the polarization of both modes ispreserved.

SUMMARY OF THE INVENTION

It is a principal object of the present invention to provide an improvedfiber which has its internal guiding region precisely located, along twoorthogonal transfer axes, relative to two external reference surfaces onthe fiber. In this connection, a related object of the invention is toprovide such an improved optical fiber which permits accurate splicingwith extremely low losses across the splices.

It is another important object of this invention to provide an improvedpolarization-holding optical fiber which can be used to form eitherco-polarized or cross-polarized couplers. Thus, a related object of theinvention is to provide such couplers which are capable of couplingsignals to and from either or both of the transverse axes of two or morefibers in a single coupler.

It is a further object of this invention to provide an improvedoptical-fiber resonator which is particularly useful in rotation sensorssuch as optical-fiber gyroscopes.

Other objects and advantages of the invention will be apparent from thefollowing detailed description in the accompanying drawings.

In accordance with the present invention, the foregoing objectives arerealized by an optical fiber comprising a core and cladding havingdifferent refractive indices and forming a single-mode guiding region,the outer surface of the fiber having a cross-section forming a pair oforthogonal exterior flat surfaces so that the location of the guidingregion can be ascertained from the geometry of the exterior surfaces ofthe fiber, and the guiding region being offset from the center ofgravity of the transverse cross-section of the fiber and locatedsufficiently close to at least one of said flat surfaces to allowcoupling to a guided wave through that surface by exposure or expansionof the field of the guiding region. In the preferred embodiment of theinvention, the core also has a non-circular cross-section defining twotransverse orthogonal axes, the core having a longer transversedimension along one of the orthogonal axes than along the other of theaxes for guiding two fundamental modes, one of the fundamental modeshaving an electric field parallel to the axis of the longer transversedimension and the other of the fundamental modes having an electricfield parallel to the axis of the shorter transverse dimension; thedifference in the core dimensions along the orthogonal transverse axesand the difference between the refractive indices of the core andcladding are sufficiently large to de-couple the fundamental modes sothat the polarization of the two modes is preserved within the fiber;the guiding region is offset from the geometric center of the fiber andlocated sufficiently close to one side of the surface of the fiber toallow coupling to a guided wave through that one side by exposure orexpansion of the field of the guiding region; and the outer surface ofthe fiber has a non-circular cross-section forming an indexing surfacewith a predetermined geometric relationship to the guiding region andthe orthogonal transverse axes so that the location of the guidingregion and the orientation of the axes can be ascertained from thegeometry of the indexing surface on the exterior of the fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, labeled PRIOR ART, is a diagrammatic perspective view, inpartial section, illustrating the electric and magnet fields in theirpreferred directions of polarization in the elliptical core of asingle-mode optical fiber waveguide;

FIG. 2 is an end view of an optical fiber waveguide according to onepreferred embodiment of the present invention;

FIG. 3 is a schematic illustration of a fiber-ring resonator utilizingthe optical fiber waveguide of this invention;

FIG. 4 illustrates a co-polarization coupler utilizing the optical fiberwaveguides of this invention;

FIG. 5 illustrates another co-polarization coupler utilizing the opticalfiber waveguides of this invention;

FIG. 6 illustrates a cross-polarization coupler utilizing the opticalfiber waveguides of this invention;

FIG. 7 illustrates a four-way coupler utilizing the optical fiberwaveguides of this invention;

FIG. 8 is a partially schematic side elevation of apparatus for drawingoptical fiber according to the present invention; and

FIG. 9 is a transverse cross-section of a unitary directional couplermade according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and will be described in detail herein. Itshould be understood, however, that it is not intended to limit theinvention to the particular forms disclosed, but, on the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theappended claims.

Turning now to FIG. 1, there is shown a dielectric core 20 forsupporting the propagation of electromagnetic fields E, H in the axialdirection. This particular core 20 has an elliptical cross-section witha major diameter 2a and a minor diameter 2b, i.e., a and b are the radiiof the core at the major and minor axes, respectively. In a single-modeoptical fiber, such a core 20 has a relatively high dielectricconstant/index of refraction which tends to confine and guideelectromagnetic energy (i.e., light) along the axis of the core. It isknown that if the index of refraction of the core 20 is properly chosenin relation to the index of refraction of the surrounding medium, thecore dimensions a, b, and the wavelength of the light, the distributionof the fields E, H will tend to occur in a single, well-defined pattern,or mode. Shown in FIG. 1 is the field pattern for the _(o) HE₁₁ mode.

Single-mode propagation has the advantage of providing well-definedfield patterns for coupling the fiber to optical devices. Anotheradvantage is that the attributes of the light propagation, such as phasevelocity and group velocity, are relatively constant as the lightpropagates down the fiber. The group velocity specifies how fastmodulation or information travels down the fiber. Thus, for transmittinginformation over long distances it is important that the group velocitybe relatively constant and in particular independent of frequency sothat the information will be localized at a specific region rather thanbecoming "smeared out" as the information travels down the fiber. Asingle constant phase velocity is important in fiber-optic sensorapplications where the phase of a signal in a sensor fiber is comparedto the phase of a reference signal in a reference fiber.

Single-mode propagation does not, however, guarantee that thepolarization of the signal is fixed in any definite or constant angularrelationship with respect to the core 20. Polarization is defined as thedirection of the electric field vector E. Thus, in the particularexample illustrated in FIG. 1, the light is polarized in a verticaldirection.

In single-mode fiber-optic sensors, the phase of the optical signal atthe end of a sensor fiber is made a function of an environmentalparameter sought to be measured. Typically this phase-shift isintroduced by physically lengthening the fiber, or by changing the indexof refraction of the core 20. But if the waveguide is notpolarization-preserving, the polarization of the light tends to changerandomly as the light propagates down the axis of the core 20. Such arandom change in polarization results in a

fluctuation of the detected signal since a 180° rotation of thedirection of polarization is equivalent, at the end of the fiber, to a180° phase shift. Thus, for sensor applications, the polarization of thelight should be maintained at a fixed angular relationship with respectto the fiber as the light propagates down the core.

To maintain or preserve the polarization of a signal in an opticalfiber, the optical properties of the fiber must be anisotropic, or inother words a function of the angle of polarization with respect to thefiber. One method of making the optical fiber anisotropic is to make thecore 20 have a cross-section which is elliptical or some othernon-circular shape which defines two transverse orthogonal axespermitting the de-coupling of waves polarized along those axes. Signalswhich are launched into such fibers in alignment with one of thetransverse axes tend to remain aligned with that axis as the signals arepropagated through the fiber, thereby preserving the polarization of thesignal.

In the illustrative embodiment of FIG. 2, an optical fiber 25 has anelliptical core 26 with a relatively high index of refraction surroundedby a cladding 27 with a lower index of refraction to produce a highdifference in index (e.g., a Δn of 0.06). The dimensions and therefractive indices of the core 26 and the cladding 27 are selected toprovide a single-mode guiding region. Because of its elliptical shapeand high index difference, this guiding region will also hold thepolarization of optical signals propagated therethrough in alignmentwith either axis of the ellipse. That is, the major and minor axes ofthe elliptical cross-section represent two transverse orthogonal axeswhich, in combination with the refractive indices of the core andcladding, de-couple light waves polarized along those axes.

Surrounding the guiding region formed by the core 26 and cladding 27 isa support layer 28 which provides the fiber with increased mechanicalstrength and ease of manipulation. Since this support layer 28 is not apart of the guiding region, its optical properties are not nearly ascritical as those of the core 26 and the cladding 27. To prevent lightfrom being trapped in the cladding 27, the support layer 28 has an indexof refraction higher than that of the cladding 27.

In accordance with one important aspect of the present invention, anoptical fiber with a guiding region having orthogonalpolarization-holding axes, i.e., axes of birefringence, is provided withan outer surface having a pair of orthogonal flat surfaces each of whichis parallel to one of the polarization-holding axes so that the locationof the guiding region and the orientation of the axes of birefringencetherein can be ascertained from the geometry of the outer surface of thefiber. Thus, in the illustrative embodiment of FIG. 2, the outer surfaceof the support layer 28 forms a first flat surface 29 parallel to themajor axis of the elliptical guiding region, and a second flat surface30 parallel to the minor axis of the elliptical guiding region. The flatsurface 29 is located sufficiently close to the guiding region thatetching away a layer Δa from the outer surface of the fiber exposes themajor axis of the elliptical guiding region in the surface 29. In thisparticular embodiment, the other flat surface 30 is spaced farther awayfrom the guiding region, by a distance Δb, so that the minor axis of theelliptical guiding region is not exposed by merely etching away thethickness Δa.

One of the significant advantages of the fiber of this invention is thatit permits precise, low-loss splices to be made. That is, the guidingregions at two similar fiber ends can be aligned with each other bysimply placing the two fiber ends on a common substrate 31 having agroove with orthogonal walls 32 and 33, with the orthogonal flatsurfaces on the two fiber ends resting against the orthogonal walls ofthe substrate. The two fiber ends are butted firmly together and joinedto each other and to the substrate by a suitable bonding orencapsulating means, preferably with the addition of a coupling agent inthe interface between the two abutting surfaces of the fiber ends. Onesuitable bonding technique is described in my copending U.S. patentapplication Ser. No. 594,478 filed Mar. 28, 1984 for "Optical-FiberDirectional Coupler Using Boron Oxide As Interstitial Material."

The common substrate can be formed by shaping an elongated glass preformto have a cross-sectional configuration with longitudinal surfaces whichare large-scale complements of the external reference surfaces on thefibers to be joined, and heating the shaped preform to a softenedcondition and then drawing the softened preform in a longitudinaldirection to reduce the scale of the complementary surfaces so thecross-sectional configuration of the drawn preform matches thecross-sectional configuration of the external reference surfaces on thefibers to be joined, and then cooling the drawn preform. The opticalfibers to be joined are then placed on the drawn preform with theexternal reference surfaces on the fibers engaging the complementarylongitudinal surfaces on the drawn preform, and are bonded to thepreform.

One application where this improved splice is particularly useful is inthe fabrication of a high-performance fiber-ring resonator using aclosed fiber ring formed by splicing the ends of a length of this fiber.With the orthogonal flat surfaces on the fiber aligned with the internalaxes of birefringence, the two ends of the fiber can be spliced withprecise alignment of the guiding regions at the two ends of the fiber.As a result, the splice has negligible loss. The closed ring formed bythis splice can then be loosely coupled to a fiber carrying the inputsignal to the ring and the output signal to a detector.

Such a fiber-ring resonator is illustrated schematically in FIG. 3 wherea laser 40 supplies an optical signal S_(o) at a frequency f_(o) throughan optical fiber 41 which forms a loop 41a. The two ends of this loop41a are coupled through a 3-dB coupler 42 which splits the signal intotwo components S₁ and S₂. These two components are propagated inopposite directions through the fiber loop 41a and respective 3-dBcouplers 43 and 44, and then on through a 10-dB coupler 45 whichprovides loose coupling to a sensing ring 46. It is this ring 46 that isformed by the fiber of the present invention, with a low-loss splice 47.

The portions of the two signals S₁ and S₂ that are coupled into the ring46 propagate around the ring in opposite directions, and any rotation ofthe ring causes a shift Δfin the frequency of both signals, in oppositedirections. That is, if the ring is rotated in the direction indicatedby the arrow 48, the signal S₁ is shifted to a frequency (f_(o) -Δf) ,and the signal S₂ is shifted to a frequency (f_(o) +Δf). Fractions ofthese shifted signals are then coupled back into the fiber loop 41athrough the 10-dB coupler 45, and propagate in opposite directionsthrough the loop to the 3-dB couplers 43 and 44. Thus, the coupler 43receives the shifted signal S₂ propagating in the opposite directionwith a frequency (f_(o) +Δf), and the coupler 44 receives the shiftedsignal S₁ propagating in the opposite direction with a frequency (f_(o)-Δf) These signals are coupled into respective detectors 49 and 50 whichconvert the optical signals into analogous electrical signals which canbe used to measure the magnitude of Δf. For example, the electricalsignals can be fed to a superheterodyne having a local oscillatorfrequency of f_(o). If desired, one of the two couplers 43 and 44 andits corresponding detector 49 or 50 could be omitted, although the useof both couplers and detectors offers the advantage of doubling thesensed frequency shift.

The fiber of this invention is also particularly useful in formingvarious types of couplers, including co-polarization and/orcross-polarization couplers. For example, FIG. 4 illustrates a pair offibers 51 and 52 arranged to couple signals polarized along the minoraxes of the elliptical cores, and FIG. 5 illustrates another pair offibers 53 and 54 arranged to couple signals polarized along the majoraxes of the elliptical cores. It will be noted that the coupler ofeither FIG. 4 or FIG. 5 can be formed from the same pair of fibers. FIG.6 illustrates yet another coupler that can be formed from a similar pairof fibers 55 and 56, this time coupling signals polarized along themajor axis of the elliptical core of one fiber 55 and the minor axis ofthe elliptical core of the fiber 56.

Three-way and four-way couplers can also be readily formed from thefiber of this invention. Thus, FIG. 7 illustrates a four-way couplerformed from four fibers 57-60, providing two co-polarization couplings(between fibers 57, 60 and fibers 58, 59) and two cross-polarizationcouplings (between fibers 57, 58 and fibers 59, 60) within a single,integral coupler.

The optical fiber of this invention is preferably made by forming apreform having the desired transverse cross-sectional configuration anddrawing an optical fiber from the preform, with the drawing rate andtemperature being controlled to produce a fiber with a cross-sectionalconfiguration similar to that of the preform. Thus, the preform can havethe same cross-sectional configuration as the fiber 35 illustrated inFIG. 2, though on a larger scale. Such a preform can be made by firstforming a cylindrical preform with an elliptical core and claddinglocated in the center thereof (using techniques known in the art), andthen grinding two adjacent sides of the preform to form a cross-sectionhaving one flat surface parallel to the major axis of the ellipticalcore and another flat surface parallel to the minor axis of theelliptical core. An optical fiber is then drawn from the ground preformat a drawing rate and temperature controlled to produce the fiber 35 ofFIG. 2, i.e., with a cross-sectional geometry substantially the same asthat of the preform but on a smaller scale.

A drawing machine suitable for precise control of the drawing process isshown in FIG. 8. In order to heat the preform to approximately thesoftening temperature, the central component of the drawing machine isan induction furnace generally designated 60 comprising an externalinduction coil 61 and an internal graphite toroid 62. The toroid 62 isapproximately 4 inches long, an inch in diameter, and has a core holeabout a quarter inch in diameter. The induction coil 61 is energized bya radio-frequency power source 63 so that the electrical heatingcurrents are induced in the graphite toroid 62, the resultingtemperature being measured by an optical pyrometer 64 and monitored by acontrol unit 65 adjusting the power source 63. In order to prevent thegraphite toroid 62 from burning, the toroid 62 is disposed within aglass cylinder 66 which is filled with a relatively inert gas such asargon from a supply 67.

A preform 68 is fed into the top of the cylinder 66 and passes throughthe center of the graphite toroid 62. The toroid 62 is heated white hot,causing the preform 62 to soften. The drawing of the fiber 69 from thepreform 68 occurs approximately at the center of the toroid 62. Thetoroid 62 has legs which stand on a support ring 72 attached to theglass cylinder 66.

The critical parameters affecting the drawing process are the feed rateV_(p) of the preform 68 toward the drawing point, the temperature at thedrawing point, and the rate V_(f) at which the fiber 69 is drawn fromthe drawing point. The temperature and rate of drawing V_(f) set thetension at which the fiber 69 is drawn. The rate of feed V_(p) of thepreform is established by a vertical linear slide generally designated74 having a lead screw driven by a drive motor 75. At the upper end ofthe slide 74 is a block 76 containing a V groove into which the top ofthe preform 68 is clamped. The rate of drawing V_(f) is established by acapstan wheel 78 below the lower end of the glass cylinder 66. The fiberis gripped between the capstan wheel 78 and a flexible plastic belt 79which is driven by a capstan motor spindle 80 and spaced by two idlerrolls 81. The fiber is then wound onto a drum 82 by a take-up mechanismconsisting of two fixed idler pulleys 83 and a movable pulley 84attached to a dancer arm 85 carrying a weight 86. The arm 85 actuates aconventional speed control for the take-up drum 82 so that fiber iswound onto the drum 82 at a tension determined by the weight 86. Thefiber is preferably oriented so that the curved surface of the fiberengages the surfaces of the capstan wheel 78 and the drum 82, so thatthe guiding region of the fiber is located on the side having the largerradius of curvature to minimize the stress on the guiding region.

In one particular example, a preform was made depositing a pure silicacladding and germania core on the inside surface of a silica tube. Thecladding and core were formed by the thermal decomposition of silicontetrachloride and germanium tetrachloride, which were circulated throughthe bore of the silica tube at approximately 1800° C. in an inductionfurnace. Diametrically opposed portions of the outside surface of thesilica tube were then ground flat, after which the tube was collapsedand lightly drawn to form a preform having an outer surface with acylindrical cross-section with a diameter of about 2.8 mm. and a centralcore and cladding of elliptical cross-section. Two adjacent sides of theelliptical-core preform were then ground flat, with the planes of theflat surfaces extending perpendicular to each other and parallel to themajor and minor axes of the elliptical core. These flat surfaces werelocated within a few thousands of an inch of the cladding. Optical fiberwas then drawn from this preform at a temperature of about 1550° C.while feeding the preform at a rate of about 0.3 mm/sec. and whilepulling fiber from the preform at a rate of about 0.5 m/sec. Theseparameters were chosen to result in a drawing tension as high aspractical without breaking the fiber. The resulting fiber had thecross-sectional configuration illustrated in FIG. 2, with an ellipticalguiding region 3.79 microns long in the direction of its major axis and1.90 microns long in the direction of its minor axis. The dimensions Δaand Δb in FIG. 2 were 7.86 microns and 5.24 microns, respectively. Theshape of the cross-section was retained as the preform was drawn into afiber due to the high drawing tension, the relatively small diameter ofthe preform, and the precise temperature and localized heating of theinduction furnace.

A unitary coupler can be formed by etching two or more of the fibers ofFIG. 2 along the desired "interaction length," i.e., the length alongwhich it is desired to couple the fields of the two fibers. The etchingis effected with a 10% concentration of hydrofluoric acid which isallowed to remain in contact with the fibers for about fifty minutes andthen removed with distilled water. The etching exposes the cladding onat least one flat side of each fiber along the desired length. Thefibers are then fed, with their flat sides facing each other, through a40-mm length of a Vycor tube having a wall thickness of 0.2 mm and witha bore sufficient to accommodate both fibers with about a five-micronclearance. The etched portions of the fibers are aligned with each otherin about the center of the Vycor tube. The fibers extending from eachend of the Vycor tube are then clamped to hold them in a stable positionwhile a vacuum of about 17 inches of mercury is applied to both ends ofthe tube. The tube is then heated until the central region of the tubeis observed to collapse onto the fibers. A four-way coupler of the typeshown in FIG. 7, formed by the method described above, is shown in FIG.9. The four fibers 57-60 are encapsulated by the surrounding Vycor tube90. The coupler can be diffusion-tuned (diffusion tuning is described inmore detail in copending U.S. application Ser. No. 755,929 assigned tothe assignee of the present invention) with the ends of two of thefibers at one end of the coupler receiving a light signal via an LED,and the ends of the fibers at the other end of the coupler connected tophotoelectric detectors with their outputs connected to separatechannels of a single chart recorder. For example, the Vycor tube can beheated to a temperature of about 1500° C. in bursts of about two secondseach. After each heat burst, the traces on the chart recorder areexamined, and the heating bursts are stopped when the traces on thechart recorder indicate the desired power-splitting ratio.

Although the invention has been described with particular reference to afiber having an elliptical guiding region, which forms orthogonal axesof birefringence by the geometry of the physical shape of the guidingregion, the invention can also be used to advantage with fibers havingstress-induced axes of birefringence. In this case, the stress appliedto the fiber to produce the birefringence must have a definiteorientation relative to the flat surfaces on the exterior of the fiberso that the internal axes of birefringence can be accurately determinedfrom the external flat surfaces The invention can also be used toadvantage with a circular-cored fiber which is not polarization-holdingbut which has a guiding region offset from the center of gravity of thetransverse cross-section of the fiber and located sufficiently close toat least one of said flat surfaces to allow coupling to a guided wavethrough that surface by exposure or expansion of the field of theguiding region by providing the fiber with a pair of orthogonal exteriorsurfaces which have a predetermined relationship to the physicallocation of the guiding region, the guiding region can be accuratelylocated from the external geometry of the fiber so that such fibers canbe spliced with a high degree of accuracy.

As can be seen from the foregoing detailed description, this inventionprovides an improved polarization-holding optical fiber which has itsinternal guiding region precisely located, along two orthogonaltransverse axes, relative to two external reference surfaces on thefiber. These external reference surfaces can also be used to preciselylocate internal axes of birefringence in a polarization-holding fiber.Because the internal guiding region can be precisely located from theexternal reference surfaces on the fiber, extremely accurate splices canbe formed with low losses across the splices. The polarization-holdingfiber can be used to form either co-polarized or cross-polarizedcouplers, with the signals being coupled to and from either or both ofthe transverse axes of birefringence, of two or more fibers in a singlecoupler. This invention also provides an improved optical-fiberresonator which is particularly useful in rotation sensors such asoptical fiber gyroscopes.

I claim as my invention:
 1. A method of forming an optical fiber jointbetween a pair of elongated optical fibers each of which has alongitudinal axis surrounded by a core and cladding having differentrefractive indices and forming a single-mode light-guiding region, thecore and cladding of each fiber having non-circular transversecross-sections defining two polarization-maintaining axes ofbirefringence transverse to said longitudinal axis of said fiber, eachof said fibers also having predetermined external flat referencesurfaces for locating the core and cladding and said axes ofbirefringence within said each fiber from the exterior geometry of thefiber, said method comprising,shaping an elongated glass preform to havea cross-sectional configuration with flat longitudinal surfacesextending along the preform length, which flat surfaces are large-scalecomplements of said external reference surfaces on the fibers to bejoined, such that said preform longitudinal surfaces will mate with saidfiber reference surfaces when said preform is drawn down to the scale ofsaid fibers, heating the shaped preform to a softened condition and thendrawing the softened preform in a longitudinal direction to reduce thescale of said complementary surfaces so that the cross-sectionalconfiguration of the drawn preform matches the cross-sectionalconfiguration of said external reference surfaces on the fibers to bejoined, and so that the preform longitudinal surfaces and the fiberreference surfaces will mate together, cooling the drawn preform andplacing the optical fibers to be joined thereon with the ends of thefibers butted together, with said external reference surfaces on thefibers matingly engaging said complementary longitudinal surfaces on thedrawn preform, and bonding said fibers to the drawn preform and bondingthe abutting ends of said fibers to each other.
 2. The method of claim 1wherein said preform and said fibers joined thereon have about the samecoefficients of thermal expansion.
 3. The method of claim 1 wherein eachof said fibers has a pair of orthogonal external reference surfaces, andsaid preform is shaped to form a longitudinal groove with orthogonalside walls so that in the drawn preform said orthogonal side walls arecomplementary to said orthogonal reference surfaces.
 4. A method offorming an optical fiber joint as set forth in claim 1 wherein thetransverse cross-section of said guiding region is symmetrical aboutonly a single transverse axis of said fiber, said exterior flat surfacesbeing orthogonal and intersecting at a point on said single transverseaxis.
 5. The method of claim 1 wherein said core has a non-circularcross-section defining two transverse orthogonal axes of birefringence,said core having a longer transverse dimension along one of saidorthogonal axes than along the other of said axes for guiding twofundamental modes of light propagation,the difference in the coredimensions along said orthogonal transverse axes and the differencebetween the refractive indices of said core and cladding beingsufficiently large to de-couple said fundamental modes so that thepolarization of said modes of light propagation is maintained within thefiber.